A retrospective evaluation of radiation dose associated with low dose FDG protocols in whole-body PET/CT

Dose estimation in patients from different protocols of 18F-FDG PET/CT studies and analysis of optimization strategies

This study aimed to evaluate the dose in different protocols of 18F-2-fluoro-2-deoxy-D-glucose positron emission tomography/computed tomography (PET/CT) procedure. The retrospective study involves 207 patients with confirmed malignancies who underwent PET/CT. Effective dose (E) from PET was estimated based on injected activity and dose coefficient as per International Commission on Radiation Protection (ICRP) 128. Estimation of E from CT was done utilizing the dose length product (DLP) method and conversion factors as per ICRP 102. There was a significant statistical difference observed in E between different PET/CT protocols (P < .001). E of PET in the whole body (WB) was found to be 4.9 ± 0.9 mSv, whereas mean volume computed tomography dose indexvol, DLP, and E of CT in WB were 7.0 ± 0.2 mGy, 674.3 ± 80.7 mGy.cm, and 10.1 ± 1.2 mSv, respectively. No linear correlation was seen between the size-specific dose estimate and E of CT (r = -0.003; P = .978). The total mean E in WB PET/CT was 17.0 ± 1.7 mSv. CT dose was contributing more than PET dose in all protocols except brain PET/CT. Optimization strategies can be evaluated only if monitored periodically.

A new upper limit for effective dose in patient administered with 18F-FDG for PET/CT whole-body imaging with diagnostic CT parameters

INTRODUCTION

The aim of present study is to estimate effective dose in patient undergoing ¹⁸F-FDG for whole body PET/CT imaging with diagnostic CT parameters and identify the lowest achievable total effective dose.

METHOD

A total of 2247 PET/CT patients with normal glucose level underwent ¹⁸F-FDG-whole body imaging procedures. The ¹⁸F-FDG dose of 3.7MBq per kg of patient weight administered via intravenous infusion. For CT parameters, kilovoltage of 140keV and current of 40 mAs were used for all studies. All the acquired images collected retrospectively and the effective dose was calculated for each patient using algorithm adapted from ICRP Publication 106, modified for patient weight and patient blood volume. The estimated effective doses were evaluated for patients' body weight and BMI.

RESULTS

The mean of total effective dose and standard deviation is approximately 15.08(4.52) mSv using ICRP algorithm. 56% of total patient has normal BMI and their average total effective dose is 13.6mSv. Underweight patients' effective dose can be as low as 9.6mSv even using diagnostic CT protocols.

CONCLUSION

The effective dose of PET/CT procedure in present study is one of the lowest although using diagnostic parameters for CT acquisition compared to published data worldwide. This is due to the improved sensitivity of PET and complex reconstruction technique that maintains the image quality. A significant association between body weight, BMI and effective dose is reported in present study. Therefore, it is suggested that attention must be given for underweight and ideal BMI patients while prescribing FDG activity and CT imaging parameters in order to minimize the effective dose. The effective dose reported in present study can be considered as an upper limit for effective dose in PET/CT patients with normal BMI. This upper limit can be treated as a standard limit when optimizing imaging parameters, developing algorithm for image reconstruction and prescribing activity for patients. This practice could fulfill ALARA principle that could reduce cancer risk.

Deep learning-based automated segmentation of eight brain anatomical regions using head CT images in PET/CT

Objective

We aim to propose a deep learning-based method of automated segmentation of eight brain anatomical regions in head computed tomography (CT) images obtained during positron emission tomography/computed tomography (PET/CT) scans. The brain regions include basal ganglia, cerebellum, hemisphere, and hippocampus, all split into left and right.

Materials and methods

We enrolled patients who underwent both PET/CT imaging (with an extra head CT scan) and magnetic resonance imaging (MRI). The segmentation of eight brain regions in CT was achieved by using convolutional neural networks (CNNs): DenseVNet and 3D U-Net. The same segmentation task in MRI was performed by using BrainSuite13, which was a public atlas label method. The mean Dice scores were used to assess the performance of the CNNs. Then, the agreement and correlation of the volumes of the eight segmented brain regions between CT and MRI methods were analyzed.

Results

18 patients were enrolled. Four of the eight brain regions obtained high mean Dice scores (> 0.90): left (0.978) and right (0.912) basal ganglia and left (0.945) and right (0.960) hemisphere. Regarding the agreement and correlation of the brain region volumes between two methods, moderate agreements were observed on the left (ICC: 0.618, 95% CI 0.242, 0.835) and right (ICC: 0.654, 95% CI 0.298, 0.853) hemisphere. Poor agreements were observed on the other regions. A moderate correlation was observed on the right hemisphere (Spearman’s rho 0.68, p = 0.0019). Lower correlations were observed on the other regions.

Conclusions

The proposed deep learning-based method performed automated segmentation of eight brain anatomical regions on head CT imaging in PET/CT. Some regions obtained high mean Dice scores and the agreement and correlation results of the segmented region volumes between two methods were moderate to poor.

Radiobiological risks in terms of effective dose and organ dose from 18F-FDG whole-body PET/CT procedures

Introduction

Integrated Positron Emission Tomography (PET) with Computerized tomography (CT) (PET/CT) are widely used to diagnose, stage and track human diseases during whole body scanning. Multi-modality imaging is an interesting area of research that aims at acquiring united morphological-functional image information for accurate diagnosing and staging of the disease. However, PET/CT procedure accompanied with high radiation dose from CT and administered radioactivity. The aim of the present study was to estimate the patients' dose from ¹⁸F-fluorodeoxyglucose imaging (18F-FDG) hybrid PET/CT whole body scan.

Materials and methods

RADAR (Radiation Dose Assessment Resource) software was used to estimate the effective dose for 156 patients (110 (70.5%)) males and 46 (39.5%) female) examined using Discovery PET/CT 710, GE Medical Systems installed at Kuwait Cancer Control Center (KCCC).

Results

The effective dose results presented in this PET/CT study ranged from (1.56-9.94 mSv). The effective dose was calculated to be 3.88 mSv in females and 3.71 mSv in males. The overall breast (female), lung, liver, kidney and thyroid were 7.4, 7.2, 5.2, 4, 3 and 2.9, respectively.For females, the body mass index (BMI) was 28.49 kg/m² and for males it was 26.50 kg/m² which showed overweight values for both genders. Conclusions: The findings indicate that the effective dose of 18F-FDG in both male and female patients was not substantially different. The study suggested that the risk-benefit proportions of any ¹⁸F-FDG whole body PET/CT scan should be clarified and carefully weighed. Patient's doses are lower compared with previous studies.

To Enhance or Not to Enhance? The Role of Contrast Medium 18F-FDG PET/CT in Recurrent Ovarian Carcinomas

Background and Objectives: ¹⁸F-fluorodeoxyglucose (FDG) positron emission tomography/X-ray computed tomography (PET/CT) represents the mainstay diagnostic procedure for suspected ovarian cancer (OC) recurrence. PET/CT can be integrated with contrast medium and in various diagnostic settings; however, the effective benefit of this procedure is still debated. We aimed to compare the diagnostic capabilities of low-dose and contrast-enhanced PET/CT (PET/ldCT and PET/ceCT) in patients with suspected ovarian cancer relapse. Materials and Methods: 122 OC patients underwent both PET/ldCT and PET/ceCT. Two groups of nuclear medicine physicians and radiologists scored the findings as positive or negative. Clinical/radiological follow-up was used as ground truth. Sensitivity, specificity, negative/positive predictive value, and accuracy were calculated at the patient and the lesion level. Results: A total of 455 and 474 lesions were identified at PET/ldCT and PET/ceCT, respectively. At the lesion level, sensitivity, specificity, positive predictive value, negative predictive value, and accuracy were not significantly different between PET/ldCT and PET/ceCT (98%, 93.3%, 97.4%, 94.9%, and 96.9% for PET/ldCT; 99%, 95.5%, 98.3%, 97%, and 98% for PET/ceCT, p = ns). At the patient level, no significant differences in these parameters were identified (e.g., p = 0.22 and p = 0.35 for accuracy, in the peritoneum and lymph nodes, respectively). Smaller peritoneal/lymph node lesions close to physiological FDG uptake sources were found in the cases of misidentification by PET/ldCT. PET/ceCT prompted a change in clinical management in four cases (3.2%) compared to PET/ldCT. Conclusions: PET/ceCT does not perform better than PET/ldCT but can occasionally clarify doubtful peritoneal findings on PET/ldCT. To avoid unnecessary dose to the patient, PET/ceCT should be excluded in selected cases.

Feasibility of equivalent performance of 3D TOF [18F]-FDG PET/CT with reduced acquisition time using clinical and semiquantitative parameters

Background

High-performance time-of-flight (TOF) positron emission tomography (PET) systems have the capability for rapid data acquisition while preserving diagnostic image quality. However, determining a reliable and clinically applicable cut-off of the acquisition time plays an important role in routine practice. This study aimed to assess the diagnostic equivalence of short acquisition time of 57 with routine 75 seconds per bed position (s/BP) of [¹⁸F]-fluoro-deoxy-glucose (FDG) PET.

Phantom studies applying EARL criteria suggested the feasibility of shortened acquisition time in routine clinical imaging by 3D TOF PET/CT scanners. Ninety-six patients with melanoma, lung or head and neck cancer underwent a standard whole-body, skull base-to-thigh or vertex-to-thigh [¹⁸F]-FDG PET/CT examination using the 3D TOF Ingenuity TF PET/CT system (Philips, Cleveland, OH). The [¹⁸F]-FDG activity applied was equal to 4MBq per kg body weight. Retrospectively, PET list-mode data were used to calculate a second PET study per patient with a reduced acquisition time of 57 s instead of routine 75 s/BP. PET/CT data were reconstructed using a 3D OSEM TOF algorithm. Blinded patient data were analysed by two nuclear medicine physicians. The number of [¹⁸F]-FDG-avid lesions per body region (head&neck, thorax, abdomen, bone, extremity) and image quality (grade 1–5) were evaluated. Semiquantitative analyses were performed by standardized uptake value (SUV) measurements using 3D volume of interests (VOI). The visual and semiquantitative diagnostic equivalence of 214 [¹⁸F]-FDG-avid lesions were analysed in the routine standard (75 s/BP) as well as the calculated PET/CT studies with short acquisition time. Statistical analyses were performed by equivalence testing and Bland–Altman plots.

Results

Lesion detection rate per patient’s body region agreed in > 98% comparing 57 s/BP and 75 s/BP datasets. Overall image quality was determined as equal or superior to 75 s in 80% and 69%, respectively. In the semiquantitative lesion-based analyses, a significant equivalence was found between the 75 s/BP and 57 s/BP PET/CT images both for SUV_max (p = 0.004) and SUV_mean (p = 0.003).

Conclusion

The results of this study demonstrate significant clinical and semiquantitative equivalence between short acquisition time of 57 s/BP and standard 75 s/BP 3D TOF [¹⁸F]-FDG PET/CT scanning, which may improve the patient’s workflow in routine practice.

CT protocol optimisation in PET/CT: a systematic review

Purpose

Currently, no consistent guidelines for CT scans used within PET/CT examinations are available. This systematic review provides an up-to-date overview of studies to answer the following questions: What are the specific CT protocols used in PET/CT? What are the possible purposes of requiring a CT study within a PET/CT scan? Is the CT protocol obtained from a dosimetric optimisation study?

Materials and method

PubMed/MEDLINE, Cochrane Library, Embase and Scopus were systematically searched for relevant studies in accordance with the PRISMA statement. The literature search was conducted from January 2007 until June 2019. Data derived from studies were standardized in order to reduce possible biases, and they were divided into clinically homogeneous subgroups (adult, child or phantom). Subsequently, we divided the CT protocol intents into 3 types (anatomic localization only, attenuation correction only and diagnostic purpose). A narrative approach was used to summarise datasets and to investigate their heterogeneity (due to medical prescription methodology) and their combination in multiseries CT protocols. When weighted computed tomography dose index (CTDI_w) was available, we calculated the volumetric computed tomography dose index (CTDI_vol) using the pitch value to make the results uniform. Eventually, the correlation between protocol intents and CTDI_vol values was obtained using a Kruskal–Wallis one-way ANOVA statistical test.

Result

Starting from a total of 1440 retrieved records, twenty-four studies were eligible for inclusion in addition to two large multicentric works that we used to compare the results. We analyzed 87 CT protocols. There was a considerable range of variation in the acquisition parameters: tube current–time product revealed to have the most variable range, which was 10–300 mAs for adults and 10–80 mAs for paediatric patients. Seventy percent of datasets presented scans acquired with tube current modulation, 9% used fixed tube current and in 21% of them, this information was not available. Dependence between mean CTDI_vol values and protocol intent was statistically significant (p = 0.002). As expected, in diagnostic protocols, there was a statistically significant difference between CTDI_vol values of with and without contrast acquisitions (11.68 mGy vs 7.99 mGy, p = 0.009). In 13 out of 87 studies, the optimisation aim was not reported; in 2 papers, a clinical protocol was used; and in 11 works, a dose optimisation protocol was applied.

Conclusions

According to this review, the dose optimisation in PET/CT exams depends heavily on the correct implementation of the CT protocol. In addition to this, considering the latest technology advances (i.e. iterative algorithms development), we suggest a periodic quality control audit to stay updated on new clinical utility modalities and to achieve a shared standardisation of clinical protocols. In conclusion, this study pointed out the necessity to better identify the specific CT protocol use within PET/CT scans, taking into account the continuous development of new technologies.

Quantitative and Qualitative Improvement of Low-Count [68Ga]Citrate and [90Y]Microspheres PET Image Reconstructions Using Block Sequential Regularized Expectation Maximization Algorithm

PURPOSE

There are several important positron emission tomography (PET) imaging scenarios that require imaging with very low photon statistics, for which both quantitative accuracy and visual quality should not be neglected. For example, PET imaging with the low photon statistics is closely related to active efforts to significantly reduce radiation exposure from radiopharmaceuticals. We investigated two examples of low-count PET imaging: (a) imaging [90Y]microsphere radioembolization that suffers the very small positron emission fraction of Y-90's decay processes, and (b) cancer imaging with [68Ga]citrate with uptake time of 3-4 half-lives, necessary for visualizing tumors. In particular, we investigated a type of penalized likelihood reconstruction algorithm, block sequential regularized expectation maximization (BSREM), for improving both image quality and quantitative accuracy of these low-count PET imaging cases.

PROCEDURES

The NEMA/IEC Body phantom filled with aqueous solution of Y-90 or Ga-68 was scanned to mimic the low-count scenarios of corresponding patient data acquisitions on a time-of-flight (TOF) PET/magnetic resonance imaging system. Contrast recovery, background variation, and signal-to-noise ratio were evaluated in different sets of count densities using both conventional TOF ordered subset expectation (TOF-OSEM) and TOF-BSREM algorithms. The regularization parameter, beta, in BSREM that controls the tradeoff between image noise and resolution was evaluated to find a value for improved confidence in image interpretation. Visual quality assessment of the images obtained from patients administered with [68Ga]citrate (n = 6) was performed. We also made preliminary visual image quality assessment for one patient with [90Y]microspheres. In Y-90 imaging, the effect of 511-keV energy window selection for minimizing the number of random events was also evaluated.

RESULTS

Quantitatively, phantom images reconstructed with TOF-BSREM showed improved contrast recovery, background variation, and signal-to-noise ratio values over images reconstructed with TOF-OSEM. Both phantom and patient studies of delayed imaging of [68Ga]citrate show that TOF-BSREM with beta = 500 gives the best tradeoff between image noise and image resolution based on visual assessment by the readers. The NEMA-IQ phantom study with [90Y]microspheres shows that the narrow energy window (460-562 keV) recovers activity concentrations in small spheres better than the regular energy window (425-650 keV) with the beta value of 2000 using the TOF-BSREM algorithm. For the images obtained from patients with [68Ga]citrate using TOF-BSREM with beta = 500, the visual analogue scale (VAS) was improved by 17 % and the Likert score was increased by 1 point on average, both in comparison to corresponding scores for images reconstructed using TOF-OSEM.

CONCLUSION

Our investigation shows that the TOF-BSREM algorithm improves the image quality and quantitative accuracy in low-count PET imaging scenarios. However, the beta value in this algorithm needed to be adjusted for each radiopharmaceutical and counting statistics at the time of scans.

EFFECTS OF THE SCAN RANGE ON RADIATION DOSE IN THE COMPUTED TOMOGRAPHY COMPONENT OF ONCOLOGY POSITRON EMISSION TOMOGRAPHY/COMPUTED TOMOGRAPHY

We performed phantom experiments to investigate radiation dose in the computed tomography component of oncology positron emission tomography/computed tomography in relation to the scan range. Computed tomography images of an anthropomorphic whole-body phantom were obtained from the head top to the feet, from the head top to the proximal thigh or from the skull base to the proximal thigh. Automatic exposure control using the posteroanterior and lateral scout images offered reasonable tube current modulation corresponding to the body thickness. However, when the posteroanterior scout alone was used, unexpectedly high current was applied in the head and upper chest. When effective dose was calculated on a region-by-region basis, it did not differ greatly irrespective of the scan range. In contrary, when effective dose was estimated simply by multiplying the scanner-derived dose-length product by a single conversion factor, estimates increased definitely with the scan range, indicating severe overestimation in whole-body imaging.

EFFECTIVE RADIATION DOSE OF 18F-FDG PET/CT: HOW MUCH DOES DIAGNOSTIC CT CONTRIBUTE?

The aim was to estimate the effective doses associated with different types of scanning protocols and how much the diagnostic computed tomography (DCT) scan contributed to the total dose of the dual-modality positron emission tomography/computed tomography (PET/CT) examinations. The results showed that an average radiation dose of 8.19 ± 0.83 mSv and 13.44 ± 5.14 mSv for the PET and CT components, respectively, resulting in a total dose of 21.64 ± 5.20 mSv. Approximately 92.7% (980 of 1057) of the patients underwent additional DCT protocols. The DCT protocols contributed 42% of the overall effective radiation doses, which was larger than the percentage contributed by the PET component (38%) and LCT protocols (20%). Reducing the diagnostic area of the DCT scans that patients undergo and decreasing the use of chest-abdomen-pelvis (CAP), abdomen-pelvis (AP) and chest DCT protocols, especially the CAP protocol, will be helpful in decreasing the effective radiation doses of PET/CT scan.

Chest Tuberculosis in Children

Chest is the commonest site of involvement by tuberculosis (TB) in children; lungs being the most frequently affected region, followed by nodes, pleura and chest wall. It is difficult to diagnose TB in children due to lack of overt symptoms and difficulty in obtaining samples for microbiological confirmation. Hence various imaging modalities play an important role in diagnostic algorithm as well as in follow-up after treatment. Standardization of chest radiograph reporting in context of clinically suspected TB is the need of the hour so as to suggest a proper diagnosis and avoid over-diagnosis. This article aims to discuss the imaging features of chest tuberculosis according to the site of involvement on various imaging modalities in the pediatric population.

Determining and updating PET/CT and SPECT/CT diagnostic reference levels: A systematic review

The aim of this systematic review is to investigate the national diagnostic reference level (NDRL) methods for positron emission tomography/computed tomography (PET/CT) and single photon emission tomography/computed tomography (SPECT/CT) procedures. A search strategy was based on the preferred, reporting items for systematic review and meta-analysis (PRISMA). Relevant articles retrieved from Medline, Scopus, Web of Science, Embase, Cinahl, and Google Scholar published up to October 2017. The search yielded 1057 articles. Fourteen articles were included in the review after a screening process. Relevant information from the selected articles were summarised and analysed. Discrepancies were found between the methodologies utilised to establish and report both PET/CT and SPECT/CT NDRLs, e.g. patient sampling and administered activity. Further research should focus on reporting more NDRLs for hybrid PET/CT and SPECT/CT examinations, and establish a robust NDRL standard for the CT portion associated with PET/CT and SPECT/CT examinations. This review provides updated NDRL reommndations to deliver more comparable international radation doses for administered activity and CT dose across PET/CT and SPECT/CT clinics.

Technologist Approach to Global Dose Optimization

Nuclear medicine technologists are specialized health professionals who cover a wide range of tasks from clinical routine (including image acquisition and processing, radiopharmaceutical dispensing and administration, patient care, and radioprotection tasks) to leading clinical research in the field of nuclear medicine. As a fundamental concern in all radiation sciences applied to medicine, protection of individuals against the harmful effects of ionizing radiation must be constantly revised and applied by the professionals involved in medical exposures. The acknowledgment that nuclear medicine technologists play a prominent role in patient management and several procedural steps, both in diagnostic and in therapeutic nuclear medicine applications, carries the duty to be trained and knowledgeable on the topic of radiation protection and dose optimization. An overview on selected topics related to dose optimization is presented in this article, reflecting the similarities and particularities of dose reduction-related principles, initiatives, and practicalities from a global perspective.

Significant dose reduction is feasible in FDG PET/CT protocols without compromising diagnostic quality

PURPOSE

To reduce the radiation dose to patients by optimizing oncological FDG PET/CT protocols.

METHODS

The baseline PET/CT protocol in our institution for oncological PET/CT examinations consisted of the administration of 5.18 MBq/kg of FDG and a CT acquisition with a reference current-time product of 120 mAs. In 2016, FDG activity was reduced to 4.44 and 3.70 MBq/kg and reference CT current-time-product was reduced to 100 and 80 mAs. 322 patients scanned with different protocols were retrospectively evaluated. For each patient, effective dose was calculated. The overall image quality was subjectively rated by the referring physician on a 4-point scale (IQ score: 1 excellent, 2 good, 3 poor but interpretable, 4 poor not interpretable). Image quality was quantitatively evaluated measuring noise in the liver.

RESULTS

CT Results: Effective dose was progressively reduced from 9.5 ± 2.8 to 8.0 ± 2.3 and 6.2 ± 1.5 mSv (p < 0.001). A mean dose reduction of 34.9% was achieved. There was a significant degradation of IQ score (p < 0.05) and noise (p < 0.001). Nevertheless, the number of poor quality studies (IQ score >2) did not increase. PET Results: Effective dose was gradually reduced from 6.5 ± 1.4 to 5.7 ± 1.3 and 5.0 ± 1.0 mSv (p < 0.001). Average dose reduction was 23.4%. IQ score (p < 0.05) and noise (p < 0.001) significantly degraded for lower activity protocols. However, all images with reduced activity were scored as interpretable (IQ score ≤ 3).

CONCLUSIONS

A significant radiation dose reduction of 28.7% was reached. Despite a slight reduction in image quality, the new regime was successfully implemented with readers reporting unchanged clinical confidence.

Effective dose estimation for oncological and neurological PET/CT procedures

BACKGROUND

The aim of this study was to retrospectively evaluate the patient effective dose (ED) for different PET/CT procedures performed with a variety of PET radiopharmaceutical compounds. PET/CT studies of 210 patients were reviewed including Torso (n = 123), Whole body (WB) (n = 36), Head and Neck Tumor (HNT) (n = 10), and Brain (n = 41) protocols with 18FDG (n = 170), 11C-CHOL (n = 10), 18FDOPA (n = 10), 11C-MET (n = 10), and 18F-florbetapir (n = 10). ED was calculated using conversion factors applied to the radiotracer activity and to the CT dose-length product.

RESULTS

Total ED (mean ± SD) for Torso-11C-CHOL, Torso-18FDG, WB-18FDG, and HNT-18FDG protocols were 13.5 ± 2.2, 16.5 ± 4.5, 20.0 ± 5.6, and 15.4 ± 2.8 mSv, respectively, where CT represented 77, 62, 69, and 63% of the protocol ED, respectively. For 18FDG, 18FDOPA, 11C-MET, and 18F-florbetapir brain PET/CT studies, ED values (mean ± SD) were 6.4 ± 0.6, 4.6 ± 0.4, 5.2 ± 0.5, and 9.1 ± 0.4 mSv, respectively, and the corresponding CT contributions were 11, 14, 23, and 26%, respectively. In 18FDG PET/CT, variations in scan length and arm position produced significant differences in CT ED (p < 0.01). For dual-time-point imaging, the CT ED (mean ± SD) for the delayed scan was 3.8 ± 1.5 mSv.

CONCLUSIONS

The mean ED for body and brain PET/CT protocols with different radiopharmaceuticals ranged between 4.6 and 20.0 mSv. The major contributor to total ED for body protocols is CT, whereas for brain studies, it is the PET radiopharmaceutical.

PET/CT: underlying physics, instrumentation, and advances

Since it was first introduced, the main goal of PET/CT has been to provide both PET and CT images with high clinical quality and to present them to radiologists and specialists in nuclear medicine as a fused, perfectly aligned image. The use of fused PET and CT images quickly became routine in clinical practice, showing the great potential of these hybrid scanners. Thanks to this success, manufacturers have gone beyond considering CT as a mere attenuation corrector for PET, concentrating instead on design high performance PET and CT scanners with more interesting features. Since the first commercial PET/CT scanner became available in 2001, both the PET component and the CT component have improved immensely. In the case of PET, faster scintillation crystals with high stopping power such as LYSO crystals have enabled more sensitive devices to be built, making it possible to reduce the number of undesired coincidence events and to use time of flight (TOF) techniques. All these advances have improved lesion detection, especially in situations with very noisy backgrounds. Iterative reconstruction methods, together with the corrections carried out during the reconstruction and the use of the point-spread function, have improved image quality. In parallel, CT instrumentation has also improved significantly, and 64- and 128-row detectors have been incorporated into the most modern PET/CT scanners. This makes it possible to obtain high quality diagnostic anatomic images in a few seconds that both enable the correction of PET attenuation and provide information for diagnosis. Furthermore, nowadays nearly all PET/CT scanners have a system that modulates the dose of radiation that the patient is exposed to in the CT study in function of the region scanned. This article reviews the underlying physics of PET and CT imaging separately, describes the changes in the instrumentation and standard protocols in a combined PET/CT system, and finally points out the most important advances in this hybrid imaging modality.

A method to assess image quality for Low-dose PET: analysis of SNR, CNR, bias and image noise

Background

Lowering injected dose will have an effect on PET image quality. In this article, we aim to investigate this effect in terms of signal-to-noise ratio (SNR) in the liver, contrast-to-noise ratio (CNR) in the lesion, bias and ensemble image noise.

Methods

We present here our method and preliminary results using tuberculosis (TB) cases. Sixteen patients who underwent ¹⁸F-FDG PET/MR scans covering the whole lung and portion of the liver were selected for the study. Reduced doses were simulated by randomly discarding events in the PET list mode data stream, and ten realizations at each simulated dose were generated and reconstructed. The volumes of interest (VOI) were delineated on the image reconstructed from the original full statistics data for each patient. Four thresholds (20, 40, 60 and 80 % of SUVmax) were used to quantify the effect of the threshold on CNR at the different count level. Image metrics were calculated for each VOI. This experiment allowed us to quantify the loss of SNR and CNR as a function of the counts in the scan, in turn related to dose injected. Reproducibility of mean and maximum standardized uptake value (SUVmean and SUVmax) measurement in the lesions was studied as standard deviation across 10 realizations.

Results

At 5 × 10⁶ counts in the scan, the average SNR in the liver in the observed samples is about 3, and the CNR is reduced to 60 % of the full statistics value. The CNR in the lesion and SNR in the liver decreased with reducing count data. The variation of CNR across the four thresholds does not significantly change until the count level of 5 × 10⁶. After correcting the factor related to subject’s weight, the square of the SNR in the liver was found to have a very good linear relationship with detected counts. Some quantitative bias appears with count reduction. At the count level of 5 × 10⁶, bias and noise in terms of SUVmean and SUVmax are up to 10 and 20 %, respectively. To keep both bias and noise less than 10 %, 5 × 10⁶ counts and 20 × 10⁶ counts were required for SUVmean and SUVmax, respectively.

Conclusions

Initial results with the given data of 16 patients diagnosed as TB demonstrated that 5 × 10⁶ counts in the scan could be sufficient to yield good images in terms of SNR, CNR, bias and noise. In the future, more work needs to be done to validate the proposed method with a larger population and lung cancer patient data.

Level of radiation dose in university hospital noninsured private health screening programs in Korea

OBJECTIVES

The aim of this study is to evaluate radiation exposure resulting from the comprehensive health examinations of selected university hospital programs and to present basic data for research and management strategies on the health effects of medical radiation exposure.

METHODS

Radiation-based diagnostic studies of the comprehensive health examination programs of ten university hospitals in Seoul, Korea, as introduced in their websites, were analyzed. The medical radiation studies of the programs were reviewed by radiologists. Only the effective doses of the basic studies were included in the analysis. The optional studies of the programs were excluded.

RESULTS

Among the 190 comprehensive health examination programs, 132 programs (69.5%) included computed tomography studies, with an average of 1.4 scans. The average effective dose of radiation by program was 3.62 mSv for an intensive program for specific diseases; 11.12 mSv for an intensive program for cancer; 18.14 mSv for a premium program; and 24.08 mSv for an overnight program. A higher cost of a programs was linked to a higher effective dose (r=0.812). The effective doses of the examination programs for the same purposes differed by as much as 2.1 times by hospital. Inclusion of positron emission tomography-computed tomography was the most critical factor in determining the level of effective dose.

CONCLUSIONS

It was found that radiation exposure dose from comprehensive health exam programs targeted for an asymptomatic, healthy public reached between 3.6 and 24 times the annual dose limit for the general public. Relevant management policies at the national level should be provided to minimize medical radiation exposure.

Value of 18F-FDG PET and PET/CT for evaluation of pediatric malignancies

Successful management of solid tumors in children requires imaging tests for accurate disease detection, characterization, and treatment monitoring. Technologic developments aim toward the creation of integrated imaging approaches that provide a comprehensive diagnosis with a single visit. These integrated diagnostic tests not only are convenient for young patients but also save direct and indirect health-care costs by streamlining procedures, minimizing hospitalizations, and minimizing lost school or work time for children and their parents. (18)F-FDG PET/CT is a highly sensitive and specific imaging modality for whole-body evaluation of pediatric malignancies. However, recent concerns about ionizing radiation exposure have led to a search for alternative imaging methods, such as whole-body MR imaging and PET/MR. As we develop new approaches for tumor staging, it is important to understand current benchmarks. This review article will synthesize the current literature on (18)F-FDG PET/CT for tumor staging in children, summarizing questions that have been solved and providing an outlook on unsolved avenues.

Dose optimization in nuclear medicine

The practice of diagnostic nuclear medicine involves the use of ionizing radiation, and thus the potential risk associated with such exposure must be weighed against the benefits to the patient. This requires that the right test with the right dose be administered to the right patient at the right time. Therefore the procedure should be performed only if it is deemed most appropriate for the clinical question being asked. If appropriate, the procedure should be performed in the most optimum manner that keeps the radiation dose to the patient as low as possible while providing the patient's clinician with information that is needed to devise a plan of medical management. If this approach is followed, the benefits to the patient will far outweigh the small potential risks associate with the procedure. This article discusses these issues, particularly in the context of cardiovascular nuclear medicine and hybrid imaging including PET/CT and SPECT/CT.

Using SUV as a guide to 18F-FDG dose reduction

This article explores how one can lower the injected (18)F-FDG dose while maintaining validity in comparing standardized uptake values (SUVs) between studies. Variations of the SUV within each lesion were examined at different acquisition times.

METHODS

Our protocol was approved by either the Human Investigation Committee or the Institutional Review Board. All 120 PET datasets were acquired continuously for 180 s per bed position in list mode and were reconstructed to obtain 30-, 60-, 90-, 120-, 150-, and 180-s-per-bed-position PET images with registration to a single set of nondiagnostic CT images. Qualitative assessment of the images was performed separately for correlation. The SUV measurements of each lesion were computed and normalized to the 180-s acquisition values to create a stabilization factor. These stabilization factors were used to demonstrate a predictable trend of stabilization over time. The variances of the stabilization factors over the entire dataset, composed of several tumor types over a range of sizes, were compared for each time point with the corresponding 150-s time point using a 2-sided F test, which has similar values to the 180-s time point.

RESULTS

The variance of the data decreased with increasing acquisition time and with increasing dose but leveled off for sufficiently long acquisitions.

CONCLUSION

Through the statistical analysis of SUVs for increasing acquisition times and visual evaluation of the plots, we developed and hereby propose an algorithm that can be used to seek the maximum reduction in administered (18)F-FDG dose while preserving the validity of SUV comparisons.

Molecular imaging in oncology: (18)F-sodium fluoride PET imaging of osseous metastatic disease

OBJECTIVE

This literature review details the history, pharmacokinetics, and utility of (18)F-sodium fluoride (Na(18)F) PET/CT in detecting osseous metastases compared with the current standard of care, technetium-99m methylene diphosphonate ((99m)Tc-MDP) bone scintigraphy. Additional discussion highlights solutions to impediments for broader implementation of this modality and insight into the complementary roles of (18)F-FDG PET/CT and Na(18)F PET/CT in oncology imaging, including preliminary data for combined Na(18)F and FDG PET/CT.

CONCLUSION

Na(18)F PET/CT is the most comprehensive imaging modality for the evaluation of osseous metastatic disease. Although further data acquisition is necessary to expand cost-benefit analyses of this imaging agent, emerging data reinforce its diagnostic advantage, suggest methods to mitigate impediments to broader utilization of Na(18)F PET/CT, and introduce a potentially viable technique for single-session combined Na(18)F and FDG PET/CT staging of soft-tissue and osseous disease.

Baffle thrombosis in an adult with remote prior scimitar vein repair mimicking massive pulmonary embolism

A 58-year-old man with a history of Scimitar syndrome diagnosed and surgically repaired in early adulthood presented multiple times to the emergency department complaining dyspnea, chest pain, and hemoptysis. Asymmetric pulmonary arterial flow rates between left and right lungs resulted in an apparent filling defect on computed tomographic pulmonary arteriography, which was repeatedly misdiagnosed clinically and radiologically as a massive pulmonary embolus. This case highlights the importance of understanding the pathophysiology and post-surgical complications of repaired congenital cardiovascular disease. Delayed phase acquisitions are often necessary to characterize the physiology of repaired congenital cardiovascular disease with associated shunts.

PET-CT in oncological patients: analysis of informal care costs in cost-benefit assessment

PURPOSE

The authors analysed the impact of nonmedical costs (travel, loss of productivity) in an economic analysis of PET-CT (positron-emission tomography-computed tomography) performed with standard contrast-enhanced CT protocols (CECT).

MATERIALS AND METHODS

From October to November 2009, a total of 100 patients referred to our institute were administered a questionnaire to evaluate the nonmedical costs of PET-CT. In addition, the medical costs (equipment maintenance and depreciation, consumables and staff) related to PET-CT performed with CECT and PET-CT with low-dose nonenhanced CT and separate CECT were also estimated.

RESULTS

The medical costs were 919.3 euro for PET-CT with separate CECT, and 801.3 euro for PET-CT with CECT. Therefore, savings of approximately 13% are possible. Moreover, savings in nonmedical costs can be achieved by reducing the number of hospital visits required by patients undergoing diagnostic imaging.

CONCLUSIONS

Nonmedical costs heavily affect patients' finances as well as having an indirect impact on national health expenditure. Our results show that PET-CT performed with standard dose CECT in a single session provides benefits in terms of both medical and nonmedical costs.

Imaging in childhood cancer: a Society for Pediatric Radiology and Children's Oncology Group Joint Task Force report

Contemporary medical imaging is a cornerstone of care for children with cancer. As 5-year survival rates for children with cancer exceed 80%, imaging technologies have evolved in parallel to include a wide array of modalities. Here, we overview the risks and benefits associated with commonly used imaging modalities and survey the current landscape of medical imaging for children with cancer. We find evidence-based imaging guidelines to assist in protocol development and to guide decision-making for optimal patient care are often lacking. The substantial variation in protocol-based recommendations for imaging both during and following therapy may hinder optimal clinical research and clinical care for children with cancer.